The law is clear in the Land of Lincoln, other than State-Regulated forms of gambling, playing a game, "Of chance or skill for money or other thing of value," is patently illegal.

With that, the State does expressly permit...
...several forms of gambling and is one of the more liberal states in that regard. They join West Virginia as being one of only a few states to have limited video lottery locations just about everywhere. It hasn’t happened yet, but the idea of legalizing and regulating online gambling through the state has also been bandied about. That would cause casinos such as Bovada, (and some others who do not operate in states that have regulated online gambling) but it hasn’t happened yet.
Sec. 28-1. Gambling.
(a) A person commits gambling when he or she:
knowingly plays a game of chance or skill for money or other thing of value, unless excepted in subsection (b) of this Section;
knowingly makes a wager upon the result of any game, contest, or any political nomination, appointment or election;
The law Paints with a Pretty Broad Brush
It would tend to include any form of online gambling that is not excepted in subsection b which, at this time, would be all online gambling.
Illinois has taken measures in the past against illegal gambling, but all of those have concerned some sort of physical location or illegal poker room in which gambling was taking place. The first offense for illegal gambling is a high-level Misdemeanor while any subsequent offense is considered a felony.
Overall...
...they are pretty tough on any unlawful gambling, but that’s probably just the state protecting its financial interests in the realm of regulated gambling. They don’t want any unauthorized gambling cutting into their revenue streams.
Worth Mentioning
We have no evidence of there ever being a charge in the state as a result of playing online as a mere player. So, while the laws may read pretty hardcore, they’re not expected to ever have a practical effect on those merely playing online.
Most online casinos that offer their games to players from the United States will also do so for players out of the State of Illinois, and here are some choices below: